Wireless communication networks are ubiquitous in many parts of the world. Advances in state of the art in communication technology, increased power and sophistication of radio network devices such as User Equipment (UE), e.g., smartphones, and concomitant increases in the complexity and data exchange requirements of user applications, all require ever-increasing bandwidth and data rates in wireless networks implementations. The Third Generation Partnership Project (3GPP) develops and promulgaes technical standards that define the protocols and requirements of wireless networks, ensuring interoperabilty geographically and between equipment makers. 3GPP has delined, and operators worldwide have deployed, a 4th generation (4G) standard known as Long Term Evolution (LTE), defined in 3GPP Technical Specification Releases 8-13. LTE includes numerous provisions to address high-bandwidth requirements, such as wider carriers (up to 20 MHz), carrier aggregation (allowing up to 100 MHz of aggregated bandwidth), multiple-antenna techniques (such as beamforming, MIMO), interference coordination (ICIC, COMP), and the like.
Current 3GPP standardization efforts relate to a 5th generation (5G) standard, referred to as New Radio (NR). NR continues and expands LTE's support for higher bandwidth and data rates by defining operations above 6 GHz, and with even broader bandwidth component carriers. Simultaneously, NR provides support for low-cost, narrowband, high-reliability, low-power, high-coverage devices, sometimes referred to as Machine-to-Machine (M2M) communications, or the Internet of Things (IoT).
In either 4G or 5G networks, a radio network device desiring to connect to a wireless communication network must acquire network synchronization (‘sync’). Network sync allows the radio network device to adjust its internal frequency relative the network, and discover the proper timing of signals received from the network. In NR, network sync will be performed using several signals.
The Primary Synchronization Signal (PSS) allows for network detection with a high frequency error, up to tens of parts per million (ppm). Additionally, PSS provides a network timing reference. 3GPP has selected mathematical constructs known as Zadoff-Chu sequences as PSS signals. One interesting property of ZC sequences is that by careful selection of two such sequences, the same correlation sequence may be used for detection, adding negligible complexity.
The Secondary Synchronization Signal (SSS) allows for more accurate frequency adjustments and channel estimation, while at the same time providing some fundamental network information, such as physical layer cell identity.
The Tertiary Synchronization Signal (TSS) provides timing information within a cell, e.g., between beams transmitted in a cell.
The Physical Broadcast Channel (PBCH) provides a subset of the minimum system information for random access.
These synchronization signals are periodically broadcast together in a System Synchronization Block (SSB). For a given transmission beam, the SSB is transmitted periodically, such as every 20 ms.
The SSB in NR will cover a larger bandwidth than in LTE. For example, the SSB may span 4.32 MHz for carrier frequencies below 6 GHZ, and may be substantially higher for carrier frequencies above 6 GHz.
The Physical Downlink Shared Channel (PDSCHSIB) provides the remaining required parts of the minimum system information necessary for a radio network device to communicate with the network; however, the PDSCHSIB is not part of the SSB. The PDSCHSIB may be transmitted in resources indicated by PBCH.
In LTE, the sync signals are located in the center of a carrier's bandwidth. Additionally, the carrier frequencies may be estimated with the help of Cell-specific Reference Signals (CRS), which are always on and allow the radio network device receiver to perform a spectral estimation to identify different carrier locations. In contrast, in NR, the SSB will be located on absolute frequency positions in a fixed (possibly band-dependent) frequency grid, unrelated to the network center frequency. Hence, in NR networks, a radio network device must assume a grid search approach for network sync.
For this reason, and additionally to minimize the use of battery power-consuming radio electronics, it is desirable for a radio network device to record as large a bandwidth as possible, and then process and analyze the signals “offline,” within the device, to identify PSS and other signal features.
Both LTE and NR use Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, and a pre-coded version of OFDM called Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. This uplink multiplexing scheme was selected to reduce the Peak to Average Power Ratio (PAPR), thus alleviating the need for very expensive and inefficient power amplifiers in radio network devices, which would otherwise be required for high-PAPR techniques, such as OFDM.
It is known and useful to utilize time/frequency domain transformations in performing sync signal detection and processing. Well-known mathematical domain transforms include the Discrete Fourier Transform (DFT) and Fast Fourier Transform (FFT). An inherent property of the DFT is that the sampling time of the signal duration on which the DFT is applied is related to the spacing between the frequency bins of the transformed signal. This frequency bin spacing is also known as subcarrier spacing (SCS) if the sampled signal is an OFDM symbol sampled according to the nominal sample rate and symbol length. Typically, a NR OFDM sync symbol has a SCS of 30 kHz., translating into a symbol length of 33.3 μs (F=1/T), excluding the cyclic prefix (CP). The number of samples, N, per nominal sample period in that DFT determines the bandwidth of the signal in the frequency domain, BW=N×SCS. The sync is transmitted less often, say every 20 ms. Per above, a 20 ms symbol sampling window length translates into a 50 Hz subcarrier spacing, and the sampling rate is chosen such that the appropriate bandwidth is received, depending on the received analog signal. Hence, the longer time translates into a more detailed frequency resolution, albeit also requiring a longer DFT. FIG. 1 depicts this relationship.
One application of the DFT is as a liter bank, where a time signal is transformed into the frequency domain. In the frequency domain it is a straightforward task to separate the desired spectral content from the undesired content. Thereafter, it is possible to inverse DFT the desired content back into the time domain. Since only a subset of the spectral content is selected, it is possible to do the IDFT with a shorter length, implying less complexity. This also implies that the time domain transformation contains fewer samples than the original time domain signal. This only reflects the fact that a more narrowband signal may be resolved with a lower sampling rate and the signal duration remains unchanged. The described process amounts to band-pass filtering in the frequency domain.
In order for a DFT to be performed efficiently, a length of a power of two is typically chosen, i.e., 2k, where k is an integer. In this case it possible to use symmetries in the DFT and realize it with an FFT using, e.g., the Radix-2 algorithm. A Radix-2 implementation typically has the complexity of N log 2 N making also very long signals affordable to transform back and forth to the frequency domain.
The de facto standard LTE sync detection algorithm is briefly described in 3GPP contribution, R1-1611899, 3GPP TSG-RAN WG1 Meeting #87, Reno Nev., 14-18 Nov. 2015. The approach described therein assumes that the presence of a mobile communications Radio Access Technology (RAT) carrier in the spectrum is detectable based on power density estimation. However, for NR where, in the case of no data to transmit, little other signaling will take place (e.g., no always-on Cell-specific Reference Signals, or CRS). Hence, a simple and rapid spectrum estimation is not possible.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.